In recent years, as control systems that control a target such as an operation valve on the basis of a detection signal received from various types of sensors in a flowmeter, air-conditioning equipment, or the like, there have been more control systems that calculate, by using a microcomputer, an amount of control or the like for a target to be controlled, eventually convert a control signal corresponding to the amount of control or the like calculated by the microcomputer into an analog signal, and output the analog signal.
Such a control system includes, for example, a voltage generating device that performs pulse width modulation (PWM) or the like on a digital signal based on a result of computation performed by the microcomputer and that converts the modulated digital signal into an analog signal by using a D/A converter so as to generate a control signal. Such a voltage generating device is required to output a linear voltage or linear current with respect to an input signal in a predetermined range. A voltage generating device in an industrial control system is required to, for example, output a voltage of 0 to 10 V or a current of 4 to 20 mA.
The output circuit disclosed in PTL 1 is known as an example of an output circuit in a final stage of a voltage generating device according to the related art. The output circuit 60 described in PTL 1 includes, as illustrated in FIG. 8, an operational amplifier U6 and a three-terminal regulator 65. The operational amplifier U6 controls a voltage at a ground terminal of the three-terminal regulator 65 on the basis of an input voltage VI supplied from a D/A converter 62 in a computer 61, thereby generating a voltage Vout equal to the input voltage VI. The output circuit 60 further includes a control device 63 that monitors the input voltage VI and a switch 64 that is formed of a contactless switch connected between an output terminal of the three-terminal regulator 65 and a load Z. If the control device 63 detects that the input voltage VI is 0 V, the switch 64 is opened so as to enable the voltage Vout to be 0 V.
Also, the output circuit disclosed in PTL 2 is known as another example of an output circuit in a voltage generating device according to the related art. The output circuit 70 described in PTL 2 includes, as illustrated in FIG. 9, an operational amplifier U8, an amplification circuit formed of a transistor Q1 and a resistor R5, and an amplification circuit formed of a transistor MP7 and a resistor R6. The operational amplifier U8 controls the transistor Q1 on the basis of an input voltage VI and thereby a voltage VOX equal to the input voltage VI is output. Although not disclosed in PTL 2, a modulation circuit 71 and a D/A converter 72 are illustrated as a circuit that generates the input voltage VI for convenience of description.
The output circuit 70 described in PTL 2 is able to output a voltage VOX of 0 V in response to a voltage VI of 0 V input to the output circuit 70.